Linear and Weakly Nonlinear Analysis of the Magneto-Rotational-Instability in Thin Keplerian Discs

The linear instability of thin, vertically-isothermal Keplerian discs, under the influence of axial magnetic field is investigated. Solutions of the stability problem are found explicitly by asymptotic expansions in the small aspect ratio of the disc. It is shown that the perturbations are decoupled into in-plane and vertical modes. Exact expressions for the growth rates as well as the number of unstable modes are derived. Those are the discrete counterpart of the continuous infinite homogeneous cylinder magnetorotational (MRI) spectrum. In addition, a weakly nonlinear analysis of the MRI is performed. It is shown that near the instability threshold the latter is saturated by the stable magnetoacoustic modes.Comment: 18 pages, 6 figures, to appear in the proceedings of the workshop "Waves and Instabilities in Space and Astrophysical plasmas" (WISAP-2011), June 201

Angular distributions of secondary electrons in fast particle-atom scattering

We present the angular distribution of electrons knocked out from an atom in a fast charge particle collision at small momentum transfer. It is determined not only by dipole but also by quadrupole transitions, the contribution of which can be considerably enhanced as compared to the case of photoionization. There the non-dipole parameters are suppressed as compared to the dipole ones by the parameter \omega R/c << 1, where is the photon energy, R is the ionized shell radius and c is the speed of light. This suppression in fast electron-atom collisions can be considerably reduced: the corresponding expansion parameter \omega R/ \nu << 1 is much bigger than in photoionization, since the speed of the incoming electron is much smaller than c. In formation of the angular distribution it is decisively important that the ionizing field in collision process is longitudinal, while in photoionization - it is transversal. We illustrate the general formulas by concrete results for outer s-, p-, and some nd-subshells of multi-electron noble gas atoms Ar, Kr and Xe, at several transferred momentum values: q=0.0, 0.1, 1.1, 2.1. Even for very small transferred momentum q, i.e. in the so-called optical limit, the deviations from the photoionization case are prominent.Comment: arXiv admin note: substantial text overlap with arXiv:1012.5465 and arXiv:1108.101

Particle acceleration in the driven relativistic reconnection

We study the compression driven magnetic reconnection in the relativistic electron-positron plasma. Making use of a 2.5D particle-in-cell code, we simulated compression of a magnetized plasma layer containing a current sheet within it. We found that the particle spectrum within the reconnecting sheet becomes non-thermal; it could be approximated by a power-law distribution with an index of -1 and an exponential cutoff.Comment: 15 pages, 9 figures, to appear in the ApJ August 10, 2008, v683n 1 issu